FIG. 1A is an illustration showing a battery housing cover mechanism 1 of a related-art mobile thermal printer. The cover mechanism 1 has an internal pivot shaft arranged at a section recessed from the outer surface of the printer body. FIG. 1B and FIG. 1C respectively show a cover in an open position and in a closed position. In FIG. 1A through 1C, there are shown a mobile thermal printer body 2, a battery housing 3, a cover member 4, shaft portions 5 of the cover member 4 and a battery B. Bearing holes 9 are formed on a recessed section 8 recessed from an outer surface 7 of a side face 6 of the printer body 2. The shaft portions 5 of the cover member 4 are fitted in the bearing holes 9, so that the cover member 4 is attached to the printer body 2.
The cover member 4 is rotated about the shaft portions 5 fitted in the bearing holes 9 to be opened as shown in FIG. 1C.5 
FIG. 2A is an illustration showing a battery housing cover mechanism 20 of another related-art mobile thermal printer. The cover mechanism 20 has an exposed pivot shaft arranged on the outer surface of the printer body. FIG. 2B and FIG. 2C respectively show a cover in an open position and in a closed position. In FIG. 2A through 2C, there are shown a mobile thermal printer body 21, a battery housing 22, a cover member 23, pierced arms 24 arranged on the outer surface of the cover member 23, and holes 25. A bearing 28 is formed on an outer surface 27 of a side face 26 of the printer body 21. A shaft member 29 passing through and supported by the bearing 28 is fitted in the holes 25, so that the cover member 23 is attached to the printer body 21.
The cover member 23 is rotated about the shaft member 29 to be opened as shown in FIG. 2C.
FIG. 3A shows a paper roll housing cover mechanism 40 of a related-art mobile thermal printer disclosed in Japanese Patent Laid-Open Publication No. 11-157170. A cover member 42 is supported on a thermal printer body 41 by a shaft 43. An operations knob 44 is provided at the center of the outer surface of the cover member 42. An open/close lever 45 integrally formed with the operations knob 44 and left and right lock levers 46 and 47 are provided on the inner surface of the cover member 42. The outer ends of the lock levers 46 and 47 are respectively fitted in recesses 48 and 49 on the inner side of side plates of the printer body 41, while pins 46a and 47a on the inner ends of the lock levers 46 and 47 are fitted in cam holes 45a and 45b of the open/close lever 45.
When the operations knob 44 is slid in the Y2 direction, the cam holes 45a and 45b move the pins 46a and 47a to move the lock levers 46 and 47 closer to each other. The lock levers 46 and 47 come out of the recesses 48 and 49 to release the lock of the cover member 42, and thus the cover member 42 is opened as shown with a chain double-dashed line. For a smooth sliding movement with less friction, grease is applied on the cam holes 45a and 45b. 
The cover mechanism 1 shown in FIG. 1A, when in the closed position, is hardly broken even if dropped and good in appearance. However, when in the open position, the cover mechanism 1 is not rigid enough.
To be more specific, since the shaft portions 5 of the cover member 4 are arranged at a section recessed from the outer surface 7 of the side face 6 of the printer body 2, the shaft portions 5 are prevented from receiving a direct impact and being broken even if a user drops the printer on the floor by mistake. Also, since the shaft portions 5 of the cover member 4 are not arranged outside the side face 6 of the printer body 2, the printer has a good appearance. However, as shown in FIG. 1C, an open angle θ1 of the cover member 4 is restricted at approximately 140 degrees by a shoulder 10 when the cover member 4 abuts thereon. Therefore, if a large force F1 for opening the cover member 4 further wider is applied when a user drops the printer on the floor during battery replacement, a larger force multiplied by leverage is generated in the shaft portions 5 and might break the shaft portions 5.
In contrast with the cover mechanism 1 shown in FIG. 1A, the cover mechanism 20 shown in FIG. 2A is hardly broken even if dropped and good in appearance when in the open position. However, when in the closed position, the cover mechanism 20 is not rigid enough to resist the damage of being dropped and has a poor appearance.
To be more specific, as shown in FIG. 2C, an open angle θ2 of the cover member 23 can be widened to 180 degrees until the cover member 23 abuts on the side face 26 of the printer body 21. Therefore, even if a force F1 is applied when a user drops the printer on the floor by mistake during battery replacement, no force is generated in the arms 24 and therefore the arms 24 are not broken. On the other hand, when the cover member 23 is in the closed position, the bearing 28 and the arms 24 are arranged outside the side face 26 of the printer body 21. Therefore, if a user drops the printer on the floor by mistake, the impact is directly transmitted to the arms 24 and might break the arms 24. Moreover, when the cover member 23 is in the closed position, the appearance is not good in that the arms 24 are arranged outside the side face 26 of the printer body 21.
In the case of the cover mechanism 40 shown in FIG. 3A, because grease is applied thereon, adhesion of dust to the grease or aging of the grease might affect the smooth operation. Also, since the grease spoils paper if penetrated thereto, it is not preferable to use grease in a printer for printing on paper.
The cover mechanism 40 is also disadvantageous in that the cover member is detachable and therefore easily lost.